Heretofore telephone companies have been reluctant to equip coin telephone sets with a data port jack that would permit customers to attach their personal computers to the telephone set because such a jack can allow certain types of fraudulent calls to be made. For example, during the initial part of a telephone coin call, the central office performs a test to see if the correct amount of money has been deposited to complete a call. This test is called the initial rate test. Initial rate is the cost of a local call. When this amount has been deposited in the phone, a connection is made between the tip and earth ground is made. The central office tests for this condition, typically by connecting battery between tip and earth ground. However, the initial rate condition can be simulated by connecting almost any part of the telephone circuitry to ground. This simulation is called "pin fraud" because one way of doing this is to force a pin into the handset cord to short the conductors to the grounded metal sheath. Another fraud problem can arise if the coin telephone has a jack to permit a customer to attach his computer modem so as to be able to send fax messages or other data over the phone line. Unfortunately, a jack that extends the tip and ring conductors of the phone to the customer's computer would also give an unscrupulous user the ability to employ the computer to simulate the tones that the coin telephone transmits when coins are deposited, and thus permit fraudulent operation of the telephone set. This type of fraud is called "red box" fraud. Notwithstanding these difficulties, it would be extremely desirable to permit customers to use their personal computers with coin telephones without subjecting the coin phone to the possibility of fraudulent usage.